A traveling wave tube and a klystron, for example, are electron tubes used for the amplification, oscillation or the like of an RF (Radio Frequency) signal by means of interaction between an electron beam emitted from an electron gun and a high-frequency circuit. As illustrated in, for example, FIG. 1, traveling wave tube (TWT) 1 includes electron gun 10 for emitting electrons, helix 20 which is a circuit for causing an electron beam formed by the electrons emitted from electron gun 10 and an RF signal to interact with each other, collector 30 for capturing electrons output from helix 20, and anode 40 for drawing electrons from electron gun 10 and guiding the electrons emitted from electron gun 10 into the helical structure of helix 20. Electron gun 10 is provided with cathode 11 for emitting electrons (thermal electrons), and heater 12 for providing thermal energy for cathode 11 to emit electrons.
Electrons emitted from electron gun 10 are accelerated by the potential difference between cathode 11 and helix 20, while forming an electron beam, and are introduced into the helical structure of helix 20. The electrons advance within the helical structure of helix 20 while interacting with an RF signal input from one end (RF in) of helix 20. Electrons having passed through the helical structure of helix 20 are captured by collector 30. At this time, an RF signal amplified by interaction with the electron beam is output from the other end (RF out) of helix 20.
Required power supply voltages are supplied from power supply device 60 to cathode 11, heater 12, anode 40 and collector 30 of traveling wave tube 1 illustrated in FIG. 1. Helix 20 is generally connected to the case of traveling wave tube 1 and grounded.
Power supply device 60 is provided with helix voltage generation circuit 61 for generating a helix voltage (Ehel) which is a negative DC voltage on the basis of the potential (HELIX) of helix 20 and supplying the helix voltage to cathode 11, collector voltage generation circuit 62 for generating a collector voltage (Ecol) which is a positive DC voltage on the basis of the potential (H/K) of cathode 11 and supplying the collector voltage to collector 30, anode voltage generation circuit 63 for generating an anode voltage (Ea) which is a positive DC voltage on the basis of the potential (H/K) of cathode 11 and supplying the anode voltage to anode 40, and heater voltage generation circuit 64 for generating a heater voltage (Ef) which is a negative DC voltage on the basis of the potential (H/K) of cathode 11 and supplying the heater voltage to heater 12.
In the traveling wave tube system of the related art illustrated in FIG. 1, the quantity of electrons emitted from cathode 11 can be controlled by the anode voltage (Ea). It is therefore possible to control the execution and stoppage of RF signal amplifying operation by traveling wave tube 1.
Note that a technique to execute or stop an RF signal amplifying operation by an electron tube by turning on or off an electron beam is also described in, for example, US Patent Application Publication No. 2011/0062898. An on-state of the electron beam refers to a state of emitting electrons from a cathode, whereas an off-state of the electron beam refers to a state of not emitting electrons from the cathode.
In the configuration described in US Patent Application Publication No. 2011/0062898 mentioned above, a first DC voltage source (for example, 1.7 kV), a second DC voltage source (for example, 4.1 kV), and a third DC voltage source (for example, 1.7 kV) connected in series are interposed between the helix and the cathode. When the electron beam is turned on, a helix voltage of 7.5 kV (=1.7 kV+4.1 kV+1.7 kV) is applied between the helix and the cathode. When the electron beam is turned off, the anode is connected to the connection node (Ea=−1.7 kV) between the first DC voltage source and the second DC voltage source and the cathode is connected to the connection node (H/K=−5.8 kV) between the second DC voltage source and the third DC voltage source to reduce the potential difference (=4.1 kV) between the cathode and the anode.
In the traveling wave tube system of the related art illustrated in FIG. 1, a method commonly used when stopping an RF signal amplifying operation by traveling wave tube 1 is to match the potential of anode 40 to the potential (H/K) of cathode 11 in order to turn off the electron beam.
In a traveling wave tube with high perveance, however, a small quantity of electrons is emitted from cathode 11, and therefore, a marginal cathode current flows as illustrated in FIG. 2, even if anode voltage generation circuit 63 is disabled to set the anode voltage (Ea) to 0 V (Ea=H/K). Accordingly, noise (thermal noise) is observed at the output terminal (RF out) of helix 20 due to the effects of the electron beam formed by the electrons. Note that methods for setting the anode voltage (Ea) to 0 V (Ea=H/K) also include changing the anode voltage (Ea) using a switch for connecting anode 40 to helix 20 or cathode 11. The anode voltage (Ea) shown in FIG. 2 represents a positive DC voltage based on the potential (H/K) of the cathode and does not show a correct voltage value.
Methods for stopping electrons from being emitted from cathode 11 include supplying a negative voltage (normally from several volts to approximately several hundred volts) to anode 40 on the basis of the potential (H/K) of cathode 11. In that case, however, positive and negative voltages need to be generated in anode voltage generation circuit 63 described above on the basis of the potential (H/K) of cathode 11, and therefore, the configuration of anode voltage generation circuit 63 becomes complicated.
Note that US Patent Application Publication No. 2011/0062898 describes an invention that assumes an electron tube provided with a focusing electrode for focusing electrons emitted from the cathode on the vicinity thereof. US Patent Application Publication No. 2011/0062898 shows that even if a potential difference of 4.1 kV is present between the cathode and the anode, the electron beam can be turned off by applying a negative voltage (=−1.7 kV with reference to the cathode potential) greater than a voltage (=1.64 kV) obtained by multiplying the potential difference by a perveance (for example, microperveance=0.4) to the focusing electrode. That is, in US Patent Application Publication No. 2011/0062898 mentioned above, there is the need for a circuit for generating a required negative voltage on the basis of the cathode potential to supply the voltage to the focusing electrode.